Burnable poison for nuclear reactor

ABSTRACT

An alloy for use as a burnable poison in nuclear reactors which consists of 0.5 - 14 weight percent gadolinium and 0 - 4 weight percent tin with the remainder being zirconium.

Unite States tent Yario May 16, 1972 54 BURNABLE POISON FOR NUCLEAR 3,362,813 1/1968 Ziolkowski 176/93 8? REACTOR OTHER PUBLICATIONS [72] Inventor: William R. Yario, West Simsbury, Conn.

[73] Assignee: Combustion Engineering, Inc., Windso Conn.

[22] Filed: Sept. 12, 1969 [21] App1.No.: 868,955

[52] U.S. Cl ..75/177, 176/93 [51] Int. Cl ..C22c 15/00 [58] Field ofSearch... ..75/177; 176/93 [56] References Cited UNITED STATES PATENTS 3,004,849 10/1961 Raine ..75/177 3,122,484 2/1964 lskenderian ....176/93 BP 3,255,092 6/1966 Dee ..176/93 BP Nuclear Science Abstracts Vol. 14, 1960, N0. 7704, pgs. 975, 976; Vol. 15, 1961 No. 29689 pg. 3,826; Vol. 17, 1963 No. 24637, pg. 3,229

AEC Research and Development Report KAPL 2162, Bibb et a1., Nov. 1, 1960 pp. 2- 12 Primary Examiner-Charles N. Lovell Atmrney Car1ton F. Bryant, Eldon H. Luther. Robert L. 0]- son, John F. Carney, Richard H. Berneike, Edward L. Kochey, Jr. and Lawrence P. Kessler [57] ABSTRACT An alloy for use as a burnable poison in nuclear reactors which consists of 0.5 14 weight percent gadolinium and O 4 weight percent tin with the remainder being zirconium.

1 Claims, N0 Drawings BURNABLE POISON FOR NUCLEAR REACTOR BACKGROUND OF THE INVENTION Commercial light water reactors employ bumable poisons additional gadolinium being present as fine particles uniformly distributed throughout the base zirconium. While the addition of gadolinium appears to have a deleterious effect on the corrosion resistance of the zirconium-gadolinium alloy, the addias a means f extending f l loading lif and permitting higher 5 tion of tin to the alloy counteracts this effect to the degree that power densities. The bumable poisons absorb neutrons and the zlrcomflmgadolmfum'tm alloy can be used wlthm the are depleted in the process. The depletion of the poison comreactor envronmem wlthout claddmg' pensates for fuel burn-up so that reactivity may remain rela- DESCRIPTION OF THE PREFERRED EMBODIMENT tively constant. Boron and boron compounds have been commonly used as poison material. Boron, however, has possible Because of the excellent poison characteristics of gadolinidisadvantages in that one helium atom and one lithium atom um as noted above, it was desired to find an efficient and are generated per boron atom burned up. Because of this economical means by which the gadolinium could be inbehavior, internal gas pressure and chemical compatibility can troduced into a nuclear reactor as a burnable poison. It was limit boron containing systems. Additionally, heating due to l 5 determined that an alloy of zirconium and gadolinium prolocal energy dispersion of alpha particles emitted upon vided the desirable structural properties of a burnable poison; neutron capture by the boron and excessive growth upon i.e., workability and solubility of gadolinium in zirconium and radiation of boron systems must be taken into account in reacuniform distribution of fine particles of the gadolinium-rich tor design. phase within the zirconium. Additionally, it was found that the When considering new materials for use as a burnable inclusion of tin in the zirconium-gadolinium alloy provided a poison, there are several important poison characteristics corrosion-resistant characteristic to the resultant alloy to perwhich must be taken into account. Firstly, the material mit its use within the reactor in an unclad condition. selected must be workable in order that it may be fabricated The zirconium-gadolinium and zirconium-gadolinium-tin into rods, sheets and tubing for inclusion in the reactor strucalloys in varying percentage compositions were prepared for ture. Secondly, the material must be readily distributable in an metallurgical and analytical evaluations by the formation of even pattern throughout the reactor to assure uniform reac- SO-gram sample buttons by the nonconsumable arc melting tivity control without creating the possibility of the developprocess. The buttons were prepared in a water-cooled copper ment of local hot spots. If the poison material is in a comhearth using a tungsten electrode with the power thereto pound or alloy form, distribution of the material within the varied from 200-600 amperes at 20 volts. In order to test the major constituent must be uniform. workability of the alloy compositions and the dispersion Poison materials in general require corrosion-resistant characteristics of the gadolinium in the zirconium, hot and cladding as protection from the corrosive action of the reactor cold working tests were performed with the results shown in water. Therefore, it would be additionally desirable that the Table l and microprobe analysis was conducted with the poison material have a corrosion-resistant characteristic so as results shown in Table II. to enable it to be used in its unclad form. If such corrosion-re- Directly from a furnace at l,550 F., the alloy buttons were sistance could be exhibited by the poison material, a signifirolled so that a reduction in thickness of 54-71 percent was cant savings in material and fabrication costs as well as an inachieved with about a 10 percent reduction accomplished per crease in reliability would result. pass. As apparent from Table l, the condition of the samples after hot working was good. The samples were then cleaned SUMMARY OF THE INVEN O and cold rolled at room temperature. Using a reduction schedule of from 5-12 percent per pass, the samples were It has been determined that the element gadolinium is an exreduced in hi k fr m 1 percent T rk y cellent burnable poison. This is because of its high neutron abh r ri tics Of the varying alloy samples over the range sorption cross section (i.e., high probability that neutrons will tested was again Show" be excfinem y the condition of the react with a nucleus so as to result in neutron capture) and the ples fter Such Col Working. The majority of the samples fact that the daughter products of the n-gamma reaction are r d i g condition with y a few a do po solid state gadolinium isotopes which do not in turn absorb iOnS exhibiting any more than insignificant edge cracking. neutrons. However, to meet the necessary structural worka- Micfopmbe analysis was utilized to determine the p bility and distribution characteristics desired in a bumable sion characteristics f the gadolinium within the zirconiumgison [he gadolinium must be incorporated in some other Compositions Of the alloy samples were observed in 20 rani l, dom areas (10 near the surface and 10 in the interior) by the A material found to provide a alloy havin the necessary microprobe beam to asses homogeneity. From the recorded bumable poison characteristics upon the incorporation of data of Table II, gadolinium was determined to be completely gadolinium therein is zirconium. Zirconium hasalow absorpl ble in Zirconium up I0 just under 3 weight percent tion cross section so as not to have any substantial effect in ga olinium. Above 3 weight percent gadolinium, a fine dark and of itself on the chain reaction; it has excellent stability gl l gadoliniumich phase wa O served randomly disupon radiation; and it shows a good resistance to corrosion in tributed throughout the particular samples. The gadoliniumreactor water environment. Gadolinium is readily soluble in rich phase was determined to be an oxide of gadolinium conzirconium up to about 3 weight percent gadolinium with any mining about 66 weight percent gadolinium.

TABLE I After hot rolling at 1,550 F. After cold rolling Composition Reductionin Reductionin Gd Sn thickness Hardness thickness Hardness (\v/o) (\v/o) (percent) (PA) Condition (percent) (PA) Condition 56 52 5o 56 Good.

2 54 53 50 57 Do. 4 60 54 41 59 Do. 6 60 50 43 58 Edge cracking. is 59 4s 3s 58 00d. 10 68 52 d 27 57 D0. 12 71 32 21a 56 Do. 14 71 48 do 2.) 56 Do.

1 1.5 55 54 d0 51 60 Minor edge cracking. 2 3.0 54 58 do 50 02 Do. 4 2.5 63 56 do 41 60 Good. 6 2.5 60 58 do 3t) 60 Minor edge cracking. 6 4.0 68 61 do i8 60 G001.

TABLE ll Summary of Microprobe Results Obtained from Zr-Gd Alloys Heat Treated at l200F for 18 Hours Nominal Microprobe Values Composition Variation Variation (w/o Gd) (w/o Gd) Comments 1 0.97 to 1.03 E No Gd-rich phase observed 2 1.88 to 2.06 +3 -45 No Gd-rich phase observed 3 2.92 to 3.1 1 :3 Small number of 66 w/o Gd particles 3.5 3.42 to 3.64 +4 2 66 w/o particles easily seen 4 3.77 to 4.24 :6 66 w/o particles easily seen 5.69 to 6.26 +4 5 66 w/o particles easily seen Range of 20 determinations assuming nominal loadings are correct in view of the above-discussed results, it can be seen that the zirconium-gadolinium alloy over varying concentrations has the fundamental desired characteristics of a good burnable poison; i.e., workability plus uniform distribution of the poison material. After considerable hot and cold working, the varying alloy combination samples remained in substantially good condition and exhibited a hardness of 32-61 Rockwell A after hot working and 56-62 Rockwell A after cold working. The microprobe analysis showed that about 3 weight percent gadolinium was soluble in zirconium and any additional gadolinium therein existed in a rich phase of fine randomly distributed particles most of which were under three microns in diameter. This arrangement is ideal for a burnable poison in that it will provide a 9 uniform and predictable poison characteristic for reactivity control without creating the possibility of locally developing hot spots.

As noted above, an additional desirable characteristic of a burnable poison is corrosion resistance. Therefore, varying alloy composition samples of zirconium and gadolinium were subjected to corrosion tests in an environment similar to that TABLE III found in an operating reactor; that is to say, the samples were immersed in water ofa chemical composition expected within a pressurized water reactor at a temperature of 680 F.

(slightly above the 650 F. maximum permitted within the reactor) and at a pressure of 2,700 psi. As noted in the tabu lated results of Table III, the addition ofgadolinium to zirconium has a marked detrimental effect on the corrosion properties ofthe zirconium.

It was determined that the addition of tin to the zirconium gadolinium alloys resulted in significant improvement in the corrosion-resistance of the new combination, this result being apparent from the data of Table III. As an illustrative example, an alloy sample having 2 weight percent gadolinium disintegrated after 100 hours of test condition while an alloy sample containing 2 weight percent gadolinium and 2.5 weight percent tin gained only 47 mg/dm. after 100 hours under the same test conditions. Upon further experimentation, it was found that annealing the as-rolled samples for 18 hours at 1,400 F. in a vacuum prior to testing resulted in still greater corrosion resistance. Where, as an illustrative example, an alloy sample having 4 weight percent gadolinium and 2.5 weight percent tin gained only 50 mg/dm. after 500 hours testing when heat treated in the above manner, a similar sample composition not heat treated gained 206 mg/dm. after 500 hours testing.

The ideal alloy composition as far as the corrosion-resistance was concerned was determined to contain on a weight percent basis a ratio of between ya-l part tin to 1 part gadolinium. A composition of the ideal ratio when heat treated in the noted manner has shown such good corrosionresistant tendencies that it appears that such a zirconiumgadolinium-tin alloy combination can be used in the reactor environment without an external cladding. This will result in considerable cost and manufacturing savings. With such combinations also meeting the criteria of workability and uniform dispersion of the gadolinium in the zirconium, the zirconiumgadolinium-tin combinations have therefore been shown to be an excellent burnable poison for nuclear reactor use.

I claim:

1. A corrosion resistant alloy for use as a burnable poison in nuclear reactors which consists of l-6 weight percent gadolinium and 0.5-4 weight percent tin with the remainder being zirconium, the ratio of tin to gadolinium on a weight basis being between 1/2 to 1 part tin to 1 part gadolinium.

Composition, \\'eight 100 hrs.

DETCEllt Specimen condition 2 300 hrs.

500 hrs.

700 hrs. Appearance Zr-LOGd.

Z1-2G(i2 5Sn do Zr-2Gd-L5Sn Sheet-HT Zr-2Gd l.5Sn Sheet-AR Zr-2Gd-2.5Sn d0 Zr-2 (l d-3.0Sn .do

Zr-3Gd-2.5Sn Sheet-AIL.

Zr-3Gd2.5Sn Sheet-HT.

Zr-4Gd-2.5Sn Sheet-AR 87 Zr-4Gd-2.5Sn Sheet-HT 32 Zircaloy-4 .do 14 White scale. D0.

Lustrous black. White flaking. Gray-brown. Dull black.

Gray-brown. Dull black. Lustrous black.

1 All specimens which contain Sn also contain 0.2 Fe and 0.1 Cr. AR-as r 3 Test terminated at this time.

4 This specimen tested at 600 F. 5 Disintegrated.

oiled; AM-as melted; HT-annealed at 1,400 F. for 18 hours in vaceuum. 

